The present invention relates to a control system for starting an electric vehicle in halt or stopping the electric vehicle in roll, and more particularly to an electric drive vehicle capable of starting and stopping without rolling backwards on sloped road and using mechanical brakes.
Description will be made on a control system for starting an electric drive vehicle on a sloped road according to conventional techniques. When the vehicle is started on a sloped road, mechanical brakes are used to stop the vehicle on the sloped road by using torque generated by the mechanical brakes. In this state, torque is output from a motor, and when it becomes possible for the motor to output torque capable of supporting the vehicle on the sloped road so as not to make the vehicle roll backwards, the mechanical brakes are released. With this operations, the vehicle can be started even on the sloped road without rolling backwards. A vehicle conducting such operations is described, for example, in U.S. Pat. No. 6,150,780 (from line 39 in 11-th column to line 20 in 20-th column).
Hill start using mechanical brakes is, however, associated with some problems because of the following reasons. First, it is difficult to effect cooperative control between mechanical brakes and a motor. For example, the following problems occur. If mechanical brakes are released before sufficient motor torque is generated, the vehicle rolls backwards. Conversely, if mechanical brakes are released immediately after sufficient torque is generated, the vehicle starts abruptly. In order to overcome these problems, the following operations and the like are required. Mechanical brakes are released at a timing when a force applied to the vehicle by gravity balances with torque output from the motor, or mechanical brakes are gradually released. It is however difficult in reality, when considering that the conditions such as a gradient of the sloped road, a vehicle weight and an operation delay of mechanical brakes change in various ways.
Second, mechanical brakes are subject to abrasion because motor torque is increased while the mechanical brakes are operated. If mechanical brakes are released at a timing when a force applied to the vehicle by gravity balances with torque output from the motor, abrasion is small. This operation is, however, difficult in reality. A generally effected operation is therefore to release mechanical brakes when the vehicle starts to roll. In this case, the vehicle starts to roll in the state that the mechanical brakes are operated, so that the mechanical brakes are subject to abrasion. Abrasion of the mechanical brakes is desired to be as small as possible because the abrasion results in an increase in a maintenance cost.
As described above, hill start using mechanical brakes is associated with difficulties in cooperative operations between the mechanical brakes and motor and abrasion of the mechanical brakes.